Experimental performance study on a dual-mode CO2 heat pump system with thermal storage

Applied Thermal Engineering 115 (2017) 393–405

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Research Paper

Experimental performance study on a dual-mode CO2 heat pump system with thermal storage Fang Liu a,⇑, Weiquan Zhu a, Yang Cai a, Eckhard A. Groll b, Jianxing Ren a, Yafeng Lei c a

College of Energy and Mechanical Engineering, Shanghai University of Electric Power, 2103 Pingliang Road, Yangpu District, Shanghai 200090, China School of Mechanical Engineering, Ray W. Herrick Laboratories, Purdue University, 177 S. Russel Street, West Lafayette, IN 47907, USA c VisionBEE, 13640 Briarwick Drive, Austin, TX 78729, USA b

h i g h l i g h t s
High compressor frequency leads to high overall COP during energy charging process.
EEV opening affects the coupled system performances significantly.
Low hot and cold water flow rates are both beneficial for the overall COP.
The overall system COP reaches up to 5.49 at certain control parameters.

a r t i c l e

i n f o

Article history: Received 30 August 2016 Revised 9 December 2016 Accepted 16 December 2016 Available online 23 December 2016 Keywords: CO2 heat pump Thermal storage Dual-mode Optimization

a b s t r a c t Performances of a water-source CO2 heat pump coupled with hot and cold thermal storage were investigated experimentally in this study. This combined system was tested by controlling compressor frequency, expansion valve opening, and hot and cold circulated water flow rates. Experimental results shows that higher compressor frequency leads to a shorter energy charging time and a higher overall coefficient of performance (COP) of the combined system during the charging process. Expansion valve opening affects the COPs significantly but affects the thermal stratification in thermal storage tanks slightly. Low hot and cold water flow rates lead to the good thermal stratification in storage tanks, which is beneficial for the overall COP of the combined system during the charging process, although high water flow rates are beneficial for the transient COP of heat pump at the beginning of the charging process. The overall COP reaches maximum when the hot and cold water flow rates were set as 0.1 m3/h and 0.2 m3/h respectively at the compressor frequency of 50 Hz and at the expansion valve opening of 330 pulses. Based on the test data, the correlations of COP with the outlet water temperature of thermal storage tanks were developed for further optimizing this combined system. Ó 2016 Elsevier Ltd. All rights reserved.

1. Introduction The electricity utility industry is undergoing massive changes. Increasing penetration levels of intermittent renewables (wind and solar power) in the energy system call for the development of Smart Grid enabling technologies. The market for energy system intelligence and flexibility – Smart Grid enabling technologies and services – becomes one of the fastest growing markets within just a few years [1]. From an energy system perspective, energy storage is the most critical for Smart Grid technology area. The specific cost of thermal storage is easily 1% or less of the costs of electrochemical or mechanical storage. Furthermore, thermal storage ⇑ Corresponding author. E-mail addresses: [email protected], [email protected] (F. Liu). http://dx.doi.org/10.1016/j.applthermaleng.2016.12.095 1359-4311/Ó 2016 Elsevier Ltd. All rights reserved.

offers advantageous characteristics, including longer life time, no degradation in capacity, and higher charge–discharge efficiency. In 2012 Blarke et al. [1] proposed to use the water source CO2 heat pump system as a ‘Thermal Battery’ with smart grid option, which has the capability of simultaneous cooling and heating, along with minimizing operational cost and CO2 emissions. This kind of thermal batteries can be applied to buildings, such as hospitals, hotels, and data centers, which require both heating (for hot water supply or space heating) and cooling (for electrical equipment or space cooling) purposes. Carbon dioxide (CO2) has environmentally friendly characteristics, ozone depletion potential (ODP) and extremely low global warming potential (GWP), and is being advocated as one of the natural refrigerants to replace CFCs and HCFCs in vapor compression systems. Study of Sarkar et al. [2] shows that the CO2 heat

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Nomenclature COP Cp EEV f GWP _ m n ODP P Q_ Q T V_

_ W

coefficient of performance (–) specific heat at constant pressure (J/kg/K) electric expansion valve compressor frequency (Hz) global warming potential mass flow rate (kg/s) pulse number (pulse) ozone depletion potential pressure (MPa) capacity (kW) overall capacity (MJ) temperature (°C) volumetric flow rate (m3/h)

power (kW)

Subscripts c cold, cold tank clg cooling comp compressor evap evaporator h hot, hot tank htg heating i inlet o outlet w water

(a) Photograph of experimental test setup

(b) Schematic of experimental test setup Fig. 1. Experimental test setup of heat pump coupled with thermal storage tanks. Table 1 Uncertainty analysis for the combined system test. Measured data

Value

Absolute uncertainty

Q_ clg (W) uncertainty

Q_ clg (W) uncertainty

COP (–) contributions

tc;o (°C) tc;i (°C) th;i (°C) th;o (°C) V_ w;c (m3/h)

15 4.29 59.46 37 0.2

0.1 0.1 0.1 0.1 0.001

43.73% 43.73% 0.00% 0.00% 12.54%

0.00% 0.00% 21.58% 21.58% 0.00%

15.01% 15.01% 3.75% 3.75% 4.30%

V_ w;h (m3/h) _ comp (W) W _ pump;c (W) W

0.1

0.0005

0.00%

56.84%

9.89%

1068

6.77

0.00%

0.00%

36.63%

52.5

2.71

0.00%

0.00%

5.87%

47.1

2.69

0.00%

0.00%

5.78%

2491 5.17 0.21%

3775 5.04 0.13%

5.367 0.05142 0.96%

_

Wpump;h (W) Calculated results Absolute uncertainty Relative uncertainty

F. Liu et al. / Applied Thermal Engineering 115 (2017) 393–405

pump system performance is significantly influenced by ambient temperature. Byrne et al. [3] and Calabrese et al. [4] found that the lower the air temperature at the gas cooler inlet, the better the performance during the heating season. Liu et al. [5] carried out an experimental study to examine the simultaneous cooling and heating performances of an ejector expansion CO2 transcritical system with an adjustable ejector and a variable speed compressor under different operating conditions, and found that the low out-

395

door air temperature benefits the coefficients of performance (COPs) while the high indoor air temperature benefits cooling and heating capacities. CO2 has a high temperature glide during exothermic process, thus when the transcritical CO2 cycle is used for water heating, the outlet water temperature of the gas cooler is relatively high and CO2 heat pump has a high heating efficiency [6]. Therefore CO2 heat pump water heaters have attracted more and more researchers and manufacturers’ interests [7–11]. Jiang

(a) COPs, cooling and heating capacities vs compressor frequency

(b) Power consumptions and refrigerant mass flow rate vs compressor frequency

(c) P-h diagram of cycles (

,

= 30 C)

Fig. 2. Impacts of compressor frequency on overall performances of the combined system and P-h diagram of cycles (n = 330 pulses, V_ w;c = 0.2 m3/h, V_ w;h = 0.4 m3/h).

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(a) Transient heating capacities

(b) Transient cooling capacities

(c) Transient compressor power consumption

(d) Transient hot water pump power consumptions Fig. 3. Impacts of compressor frequency on transient performances of the combined system (n = 330 pulses, V_ w;c = 0.2 m3/h, V_ w;h = 0.4 m3/h).

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397

(e) Transient cold water pump power consumptions

(f) Transient heating COPs

(g) Transient cooling COPs Fig. 3 (continued)

et al. [7] studied the transcritical CO2 water to water heat pump experimentally and found that the heating COP decreases as the outlet water temperature of the gas cooler increases. Yokoyama et al. [12] analyzed the performance of a water heating system composed of a CO2 heat pump and a hot water storage tank by numerical simulation, and found that the daily change in the hot water demand does not significantly affect the daily averages of the COP, and storage and system efficiencies. Fornasieri et al. [13] has proved the necessity of optimized design and control strategy of CO2 air/water heat pump systems. Minetto [14] developed a new control method for the upper cycle pressure to maximize the COP of the CO2 heat pump combined with a thermal storage tank for domestic hot water. Wang et al. [15] investigated the dynamic COP of a thermal battery system as a function of different gas cooler pressures, water flow rates and expansion valve openings, and found that a 20% more efficient in terms of cooler capacity can be achieved by controlling the hot and cold water flow rates. Jensen et al. [16] developed a dynamic model for a heat pump system including hot and cold thermal storages for flexible and simultaneous supply of heating and cooling for buildings, and their simulation results show

(a) Comparison of transient hot and cold tank temperature gradients at various compressor frequencies Fig. 4. Impacts of compressor frequency on the water temperatures of thermal storage tanks (n = 330 pulses, V_ w;c = 0.2 m3/h, V_ w;h = 0.4 m3/h).

that the performance of the heat pump is highly sensitive to the temperature distribution in the storages. However, there are very few investigation on the influence of compressor frequency on the performances of a dual-mode CO2 heat pump coupled with hot and cold thermal storage tanks in the literature. Furthermore, there is a lack of empirical correlations of COP for optimizing a heat pump with thermal storage in the literature. Therefore, the objectives of this study are (a) to investigate

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cold-side water flow rates; (b) to develop the correlation of COP for optimizing the system performances of heat pump coupled with thermal storage during the charging process of thermal storage tanks. This study will be helpful for the performance optimization of transcritical CO2 heat pump system coupled with both hot and cold thermal storage. 2. Experiments 2.1. Experimental setup

(b) Transient inlet and outlet water temperature of hot tank

(c) Transient inlet and outlet water temperature of cold tank Fig. 4 (continued)

In order to investigate the performances of a dual-mode CO2 heat pump with thermal storage during the energy charging process, tests was carried out on a 3 kW transcritical CO2 heat pump system coupled with hot and cold thermal storage tanks by controlling compressor frequency, expansion valve openings, hotside and cold-side circulated water flow rates. Fig. 1 shows a schematic of the heat pump coupled with thermal storage, which consists of a single-stage variable frequency compressor, a pulse controlled expansion valve, an evaporator, a gas cooler, an internal heat exchanger, a cold water thermal storage tank with a variable frequency pump, and a hot water thermal storage tank with a variable frequency pump. A solenoid actuator is provided for effecting movement of the expansion valve member from its closed to its open position under energization of the solenoid actuator (onvalve pulse: 32 ± 20pulses, full stroke pulses: 500 pulses). Water volumes in the hot and cold thermal storage tanks are different from each other to maintain appropriate energy balance (163 L cold tank, 176 L hot tank). In the hot tank, water is pumped out from the bottom into the gas cooler and flows back into the tank at the top. In the cold tank, water is pumped out from the top of the tank and flows back to the bottom of the tank. 2.2. Experimental instrumentation and test runs

(a) Transient COPs vs outlet water temperature of hot tank

(b) Transient COPs vs outlet water temperature of cold tank Fig. 5. Impacts of compressor frequency on transient COPs of the combined system vs outlet water temperature of tanks (n = 330 pulses, V_ w;c = 0.2 m3/h, V_ w;h = 0.4 m3/ h).

experimentally the performances of a transcritical CO2 heat pump coupled with hot and cold thermal storage by controlling compressor frequency, expansion valve openings, hot-side and

Twenty thermocouples with ±0.1 °C accuracy were installed to measure the temperatures of refrigerant-side and water-side of the combined system; among them five thermocouples were installed in each thermal storage tank as shown in Fig. 1. Four absolute pressure sensors with a full-scale accuracy of ±0.1% were installed to measure the refrigerant-side pressures. One mass flow meter with a full-scale accuracy of ±0.2% was installed to measure the refrigerant mass flow rate through the compression cycle. Two volumetric flow meters with a full-scale accuracy of ±0.5% were installed to measure the hot and cold circulated water flow rates. Three power meters with an accuracy of ±0.4% readings plus 0.1% full-scale were installed to measure the electrical power consumptions of the compressor, the hot and cold water circulation pumps separately. An Agilent 34970A data acquisition system was used to convert the incoming voltages from the measuring instrumentation to digital signals and then to transfer the signals to a personal computer, and then a real-time software program was used for data analysis. Parametric studies on the performances of this combined system during the energy charging process were carried out with 44 test runs. Through the control panel, the compressor frequency f was set at 35 Hz, 40 Hz, 45 Hz, 50 Hz, a pulse controlled expansion valve opening n was set at 240 pulses, 270 pulses, 300 pulses, 330 pulses, 350 pulses, the volumetric flow rate of the variable frequency pump for hot water circulation V_ w;h was set at 0.1 m3/h, 0.2 m3/h, 0.3 m3/h, 0.4 m3/h, 0.5 m3/h, and the volumetric flow rate of the variable frequency pump for cold water V_ w;c was set at 0.2 m3/h, 0.3 m3/h, 0.4 m3/h, 0.5 m3/h. The accuracy of the control parameter, compressor frequency, electric expansion valve (EEV) opening and hot/cold water flow rates are within ±0.1 Hz, ±1 pulse

F. Liu et al. / Applied Thermal Engineering 115 (2017) 393–405

and ±0.01 m3/h respectively. The initial water temperature in both thermal storage tanks is set as 27 °C. The thermal energy storage tanks were being charged until an average water temperature in

399

hot tank increases up to 60 °C. The measurement of the transient pressures, temperatures, flow rates and power consumptions were recorded every 5 seconds.

(a) Overall heating COPs vs EEV opening and hot cold water flow rate

(b) Overall cooling COPs vs EEV opening and hot cold water flow rate

(c) Compressor work vs EEV opening and hot cold water flow rate

(d) Refrigerant mass flow rate vs EEV opening and hot cold water flow rate Fig. 6. Impacts of EEV opening on overall performances of the combined system (f = 50 Hz, V_ w;c = 0.2 m3/h, V_ w;h = 0.1 m3/h).

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2.3. Data reduction of experimental setup tests The transient water-side heating and cooling capacities and the total power consumption were calculated by Eqs. (1)–(5) respectively.

Cooling capacity : Q_ clg ¼ q  V_ w;c  C p  ðtc;o  t c;i Þ

ð1Þ

Heating capacity : Q_ htg ¼ q  V_ w;h  C p  ðth;o  t h;i Þ

ð2Þ

_ tot ¼ W _ comp þ W _ pump;c þ W _ pump;h Total power : W

ð3Þ

Overall cooling capacity : Q clg;ov erall ¼

n X Q_ clg;i

ð4Þ

i¼0

Overall heating capacity : Q htg;ov erall ¼

n X Q_ htg;i

ð5Þ

heating capacity and the overall cooling capacity are around 25 MJ and 13 MJ respectively (Fig. 2(a)). As compressor frequency increases from 35 Hz to 50 Hz, the refrigerant mass flow rate increases and the enthalpy differences through gas cooler and evaporator both increase, thus transient heating and cooling capacities both increase, which results in a shorter energy charging time (Figs. 2(b), (c) and 3(a), (b)); thus the overall power consumptions for compressor and water circulation pumps decreases (Figs. 2(b) and 3(c)), the overall cooling capacity varies slightly and the overall heating capacity drops slightly due to the shorter charging time (Fig. 2(a)). Fig. 2(c) shows the enthalpy difference across the compressor increases as compressor frequency increases, which leads to the increase in transient power consumption of the compressor (Fig. 3(c)), the transient hot water pump power consumption at 50 Hz is lower than that at lower compressor frequency (Fig. 3 (d)), while transient cold water pump power consumptions are almost the same for different compressor frequencies (Fig. 3(e)).

i¼0

The water-side system COPs were calculated by Eqs. (6)–(9).

Cooling COP : COP clg ¼

Q_ clg _ tot W

ð6Þ

Heating COP : COP htg ¼

Q_ htg _ tot W

ð7Þ

Total COP : COP ¼

Q_ clg þ Q_ htg _ tot W Pn

Overall COP : COP ov erall ¼

ð8Þ _

_

i¼0 ðQ clg;i þ Q htg;i Þ Pn _ i¼0 ðW tot;i Þ

ð9Þ

(a) Transient heating capacities

where q is 1000 kg/m3, Cp is 4186.8 J/(kg °C), i is record number during the energy charging process, t c;i and t c;o are the inlet and outlet water temperatures of cold thermal storage tank respectively, t h;i and t h;o are the inlet and outlet water temperatures of hot thermal storage tank respectively. 2.4. Uncertainty analysis of the combined system measurements Table 1 lists the measured parameters that were used to determine the transient heating and cooling capacities and the transient system COP. For each parameter, the measured values and the absolute uncertainties are listed as well. In addition, Table 1 presents the calculated heating capacity, cooling capacity and total COP based on the measured values in a transient state. The uncertainties of the capacities and the total COP were determined using a standard error analysis in EES [17] based on the uncertainties of the individual measurements. It can be seen that the transient cooling capacity, heat capacity and COP can be measured within ±0.21%, ±0.13% and ±0.96% respectively given the listed accuracy of the various measurement instrumentations. It can also be seen that the measurement uncertainty of the compressor power, of ±6.77 W, has the most significant contribution to the final uncertainty of the calculated system transient COP.

(b) Transient cooling capacities

3. Experimental results and discussion 3.1. Impacts of compressor frequency on system performances During the thermal energy charging process at compressor frequency of 35 Hz, 40 Hz, 45 Hz, 50 Hz, expansion valve opening was set at 330 pulses, cold water flow rate was set at 0.2 m3/h and hot water flow rate was set at 0.4 m3/h, the average water temperature in hot tank was heated from 27 °C to 60 °C, the overall

(c) P-h diagram of cycles (

,

= 30°C)

Fig. 7. Impacts of EEV opening on transient heating and cooling capacities of the combined system and P-h diagram of cycles (f = 50 Hz, V_ w;c = 0.2 m3/h, V_ w;h = 0.1 m3/h).

F. Liu et al. / Applied Thermal Engineering 115 (2017) 393–405

The transient heating and cooling COPs at 50 Hz dropped more quickly than those at lower compressor frequency (Fig. 3(f) and (g)). Fig. 4(a) shows that as compressor frequency increases, thermal stratifications in hot and cold water tanks both become more obvious. This is because higher compressor frequency leads to higher transient heating and cooling capacities (Fig. 3(a) and (b)), which causes the inlet water temperature of hot/cold water storage tank increases/decreases faster than that at lower compressor frequency as shown in (Fig. 4(b) and (c)). During the energy charging process, the transient COP decreases as the outlet water temperature of hot tank increases while increases as the outlet water temperature of cold tank increases (Fig. 5(a) and (b)). The performance of the heat pump is highly sensitive to the outlet water temperatures of thermal storage tanks.

3.2. Impacts of EEV opening on system performances During the thermal energy charging process at EEV opening from 240 pulses, 270 pulses, 300 pulses, 330 pulses, 350 pulses,

(a) Transient compressor power consumption

(c) Transient cold water pump power consumptions

401

compressor frequency was set at 50 Hz, cold water flow rate V_ w;c was set at 0.2 m3/h and the hot-side water flow rate V_ w;h was set at 0.1 m3/h. Fig. 6(a) and (b) present a three-dimensional plot of heating and cooling COPs as a function of EEV opening and hotside water flow rate respectively. It was found that heating and cooling COPs both varied with expansion valve opening significantly at fixed hot and cold water flow rates; heating and cooling COPs reach maxima, 3.23 and 2.26, respectively, at the EEV opening of 330 pulses (Fig. 6(a) and (b)). This is owing to the shortest thermal energy charging time and the least overall compressor power consumption during the energy charging process at EEV opening of 330 pulses (Figs. 6(c) and 7(a), (b)). The shortest thermal energy charging time at EEV opening of 330 pulses results from the relatively high transient heating capacity, which is owing to the relatively high average refrigerant mass flow rate (Figs. 6 (d) and 7(a)). As EEV opening was increased from 240 pulses to 350 pulses, the compressor discharge pressure decreases from 8.5 MPa to 8.0 MPa (Fig. 7(c)). EEV opening doesn’t affect the enthalpy difference through the compressor very much but affect

(b) Transient hot water pump power consumptions

(d) Transient heating COPs at different EEV opening

(e) Transient cooling COPs at different EEV opening Fig. 8. Impacts of EEV opening on transient power consumptions and COPs of the combined system (f = 50 Hz, V_ w;c = 0.2 m3/h, V_ w;h = 0.1 m3/h).

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(a)

Comparison of transient temperature gradients in thermal storage tanks at different EEV opening

(b) Transient inlet/outlet refrigerant temperature of evaporator, transient inlet/outlet water temperature of cold tank Fig. 9. Impacts of EEV opening on the performances of thermal storage tanks (f = 50 Hz, V_ w;c = 0.2 m3/h, V_ w;h = 0.1 m3/h).

the average refrigerant mass flow rate and the overall compressor work significantly (Figs. 7(c) and 6(c), (d)). The relatively high transient heating COPs at EEV opening of 330 pulses benefits from the relative high heating capacity and the lower cold water pump power consumption during the charging process; the relatively high transient cooling COPs at EEV opening of 330 pulses during the later stage of the charging process benefits from the relative high cooling capacity and the lower cold water pump power consumption (Figs. 7(a), (b) and 8(a)–(e)). EEV opening also doesn’t affect thermal stratification in thermal storage tanks significantly as shown in Fig. 9(a). During the energy charging process, when the charging time is between 3000 s and 4000 s, the water temperatures Tc,1 to Tc,5 through the cold tank dropped around 15 °C at the same time as indicated by five thermocouples, and the temperature gradient through the cold tank became almost zero (Fig. 9(a)); the evaporator outlet temperature and the evaporator average temperature dropped suddenly, which caused the inlet and outlet water temperatures of the cold tank dropped, thus then thermal stratification appeared again in the cold tank (Fig. 9(a) and (b)). The cooling COP also dropped at that time as the temperature difference through the evaporator dropped, which was caused by the drop of the refrigerant-side temperature difference through the evaporator (Fig. 9(b)). A sudden drop in cooling COP could be prevented if the aspect ratio and the size of cold tank are optimized.

3.3. Impacts of hot and cold water flow rates on system performances During the thermal energy charging process at various hot water flow rate of 0.1, 0.2, 0.3, 0.4 or 0.5 m3/h and various cold

water flow rate of 0.2, 0.3, 0.4 or 0.5 m3/h, EEV opening was fixed at 330 pulses and compressor frequency was set at 50 Hz. Fig. 10(a) presents a three-dimensional plot of the heating/cooling COP as a function of the hot-side water flow rate and the cold-side water flow rate. It was observed that heating and cooling COPs both varied with the hot-side water flow rate and the cold-side water flow rate. Heating and cooling overall COPs both reached maxima, 3.23 and 2.26, respectively, and the total COP reached 5.49 during the charging process, at the cold water flow rate V_ w;c of 0.2 m3/h and the hot-side water flow rate V_ w;h of 0.1 m3/h. This is owing to the shortest charge time and the best thermal stratification caused by low hot and low cold water flow rates (Figs. 10 (b) and 11(a)). Higher flow rates (e.g. 0.5 m3/h) lead to a smaller temperature gradient in thermal storage tanks as shown in Fig. 11(a), thus the outlet water temperature of hot tank increases quickly (Fig. 11(b)) and the transient heating/cooling COPs decreases quickly (Fig. 10(b)), which results in a lower overall heating COP and a lower overall cooling COP (Fig. 10(a)); high transient heating capacity at low hot and cold water flow rates (V_ w;h of 0.1 m3/h, V_ w;c of 0.2 m3/h) led to a short charging time (Fig. 10(c)). Fig. 11(b) shows that the inlet/outlet water temperature of hot tank at lower water flow rate is higher/lower than that at higher water flow rate; however lower water flow rate benefits the thermal stratification of thermal storage, and the outlet water temperature of hot tank at lower flow rate increases pretty slower than those at higher flow rate, which can prevent the transient COP of heat pump cycle dropping quickly until the end of the charging process (Fig. 10(b)). Thermal stratification is one of the most important impact factors on the heat pump system COPs, and the condenser outlet water temperature should be kept low as long

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(a) Heating/cooling overall COPs vs hot and cold water flow rates

(b) Transient heating and cooling COP at different hot water flow rates

(c) Transient heating and cooling capacities Fig. 10. Impacts of circulated water flow rates on the combined system performances (f = 50 Hz, n = 330 pulses).

as possible. The overall COP of the heat pump system combined with thermal storage at low hot and low cold water flow rates is higher than those at higher circulated water flow rates.

storage during the energy charging process, the following correlation equation of the transient total COP with the outlet water temperatures of hot and cold thermal storage tanks was developed based on the test data using EES [17].

3.4. Optimization of the combined system during the energy charging process

COPmax ¼ 10  0:001287th;o  0:001048t2h;o  0:3754tc;o

Based on the above system performance analysis, it is found that during the energy charging process, the overall COPs of heat pump coupled with thermal storage reach maxima at compressor frequency of 50 Hz, EEV opening of 330 pulses, hot water flow rate of 0.1 m3/h, cold water flow rate of 0.2 m3/h (Fig. 6(a) and (b)). The overall system power input for is 6.94 MJ, the cooling thermal energy storage is 15.67 MJ, the heating thermal energy storage is 22.41 MJ, the cooling COP is 2.26, the heating COP is 3.23, the overall system COP is 5.49 and the thermal energy charging time is 97 min. This is owing to the better thermal stratification of thermal storage tanks and the shorter thermal energy charging time. The outlet water temperatures of thermal storage tanks affect the COPs of heat pump cycle significantly and reflect the thermal stratification in thermal storage tank. In order to further optimize the system performances of the heat pump coupled with thermal

þ 0:01015t2c;o ð27  C < th;o < 60  C;0  C < tc;o < 27  C; R2 ¼ 96:76%Þ

ð10Þ

4. Summary Performances of a transcritical CO2 heat pump system coupled with hot and cold thermal storage tanks during the energy charging process were investigated experimentally in this study. It was found that the system performance optimization of a dual-mode CO2 heat pump coupled with hot and cold thermal storage can be achieved through controlling compressor frequency, expansion valve opening and hot and cold water flow rates during the energy charging process of thermal storage tanks. The findings throughout the investigations are as follows.

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(a) Transient temperature gradients in thermal storage tank at different hot/cold water flow rates

(b) Transient inlet/outlet water temperature of hot tank at different hot water flow rates ( m3/h)

,

= 0.2

Fig. 11. Impacts of circulated water flow rates on the performances of thermal storage tanks (f = 50 Hz, n = 330 pulses).

(1) Higher compressor frequency leads to larger transient heating and cooling capacity, better thermal stratification as well as shorter charging time, which results in the better overall COP during the charging process. (2) Expansion valve opening doesn’t affect the thermal stratification in thermal storage tank significantly, but affects the refrigerant mass flow rate, the heating and cooling capacities, the compressor work as well as the system COPs significantly. (3) Lower circulated water flow rates lead to better thermal stratification, which is one of the most important impacts on the system performances of heat pump with thermal storage during the charging process, since the inlet water temperatures of gas cooler and evaporator, i.e. the outlet water temperatures of thermal storage tanks, affect the system COP significantly. Low hot and cold water flow rates are both beneficial for the overall COPs. (4) Experimental performance analysis of the heat pump system with thermal storage during the energy charging process shows that the overall system COPs reaches maxima at the fixed compressor frequency of 50 Hz, expansion valve opening of 330 pulses, cold water flow rate of 0.2 m3/h s and hot water flow rate of 0.1 m3/h; the overall power consumptions for compressor, hot and cold water circulation pumps all reach minima, and the total value is 6.94 MJ; the cooling thermal energy storage is 15.67 MJ, the heating thermal energy storage is 22.41 MJ, the cooling COP is 2.26, the heating COP is 3.23, the overall system COP is 5.49 and the thermal energy charging time is 97 min. The correlation of the system transient COP with the outlet water temperature of thermal storage tanks were developed using the test data in order to further optimize the overall performances of the combined system during thermal energy charging process. The multivariable optimal control strategies of the combined system will be studied in the future.

Acknowledgements The financial supports from the Natural Science Foundation of Shanghai in China (Grant No. 15ZR1417700), the Program for Professor of Special Appointment (Eastern Scholar) supported by Shanghai Institutions of Higher Learning (2013-66), and ‘‘Shuguang program” supported by Shanghai Education Development Foundation and Shanghai Municipal Education Commission in China (14SG50) are gratefully acknowledged.

References [1] M.B. Blarke, K. Yazawa, A. Shakouri, C. Carmo, Thermal battery with CO2 compression heat pump: techno-economic optimization of a high-efficiency smart grid option for buildings, Energy Build. 50 (2012) 128–138. [2] J. Sarkar, S. Bhattacharyya, M. Ramgopal, Performance of a transcritical CO2 heat pump for simultaneous water cooling and heating, Int. J. Appl. Sci., Eng. Technol. 6 (2010) 57–63. [3] P. Byrne, J. Miriel, Y. Lenat, Design and simulation of a heat pump for simultaneous heating and cooling using HFC or CO2 as a working fluid, Int. J. Refrig. 32 (2009) 1711–1723. [4] N. Calabrese, R. Mastrullo, A.W. Mauro, P. Rovella, M. Tammaro, Performance analysis of a rooftop, air-to-air heat pump working with CO2, Appl. Therm. Eng. 75 (2015) 1046–1054. [5] F. Liu, E.A. Groll, J. Ren, Comprehensive experimental performance analyses of an ejector expansion transcritical CO2 system, Appl. Therm. Eng. 98 (2016) 1061–1069. [6] Y. Ma, Z. Liu, H. Tian, A review of transcritical carbon dioxide heat pump and refrigeration cycles, Energy 55 (2013) 156–172. [7] Y. Jiang, Y. Ma, M. Li, L. Fu, An experimental study of trans-critical CO2 water– water heat pump using compact tube-in-tube heat exchangers, Energy Convers. Manage. 76 (2013) 92–100. [8] A. Arteconi, E. Ciarrocchi, F. Polonara, J.F. Chen, S. Deng, R.Z. Wang, Performance analysis of a carbon dioxide heat pump installed in a residential application, in: Proceedings of 11th IIR Gustav Lorentzen Conference on Natural Working Fluids, ID16, Hangzhou, China, 2014. [9] F.Z. Zhang, P.X. Jiang, Y.H. Zhu, S.S. Sun, System optimization of CO2 heat pump water heater based on economy, in: Proceedings of 11th IIR Gustav Lorentzen Conference on Natural Working Fluids, ID125, Hangzhou, China, 2014. [10] H.M. Qi, Q. Chen, J. Wei, L.M. Tang, G.M. Chen, Development and validation of variable frequency transcritical CO2 heat pump systems, in: Proceedings of

F. Liu et al. / Applied Thermal Engineering 115 (2017) 393–405 11th IIR Gustav Lorentzen Conference on Natural Working Fluids, ID160, Hangzhou, China, 2014. [11] P.C. Qi, Y.L. He, X.L. Wang, X.Z. Meng, Experimental investigation of the optimal heat rejection pressure for a transcritical CO2 heat pump water heater, Appl. Therm. Eng. 56 (2013) 120–125. [12] R. Yokoyama, T. Wakui, J. Kamakari, K. Takemura, Performance analysis of a CO2 heat pump water heating system under a daily change in a standardized demand, Energy 35 (2010) 718–728. [13] E. Fornasieri, S. Girotto, S. Minetto, CO2 heat pump for domestic hot water, in: 8th IIR Gustav Lorentzen Conf. on Natural Working Fluids, Copenhagen, 7–10, September, 2008.

405

[14] S. Minetto, Theoretical and experimental analysis of a CO2 heat pump for domestic hot water, Int. J. Refrig. 34 (2011) 742–751. [15] T. Wang, S. Dharkar, O. Kurtulus, E.A. Groll, K. Yazawa, Experimental study of a CO2 thermal battery for simultaneous cooling and heating applications, in: Proceedings of 15th Int. Refrigeration and Air Conditioning Conference at Purdue, West Lafayette, IN, USA, 2014, R2701. [16] L.H. Jensen, A. Holten, M.B. Blarke, E.A. Groll, A. Shakouri, K. Yazawa, Dynamic analysis of a dual-mode CO2 heat pump with both hot and cold thermal storage, in: Proceedings of the ASME 2013 International Mechanical Engineering Congress and Exposition, San Diego, California, USA, 2013, IMECE2013-62894. [17] S.A. Klein, 2004, Engineering Equation Solver, F-Chart Software.